Since the 1800s, gasoline engines have largely been operated by (1) controlling power output with a throttle that restricts airflow, (2) using a simple spark to control burn timing, and (3) operating close to fuel-air stoichiometry for reliable spark ignition and so catalysts can reduce NOx, HC, and CO emissions. The throttle hurts fuel efficiency with pumping losses (especially at low-load), and the stoichiometric mixtures used are thermodynamically less fuel efficient than mixtures diluted with air or exhaust gases.
With the broad availability of enabling technologies (e.g. variable valve timing), a relatively new type of combustion called homogeneous charge compression ignition (HCCI) has received increased research interest over the past decade. HCCI uses autoignition to burn lean (excess air) mixtures and can produce ultra-low NOx quantities that do not require expensive catalyst aftertreatment. Instead of a spark, combustion timing is controlled by the thermodynamic trajectory of the mixture and complex chemical kinetics. With both ultra-low NOx production and freedom from the stoichiometric shackles of spark ignition, HCCI, achieves greater fuel efficiency through thermodynamically ideal lean mixtures and unthrottled operation. This improved fuel economy, has real-world relevance to near-term sustainability, national oil independence, and greenhouse gas initiatives that seek to curb petroleum usage.
The primary challenge of HCCI autoignition is to ensure that the burn timing is synchronized against the motion of the piston varying the cylinder volume as function of crank angle. These angles are measured relative to when the piston is at the top of the cylinder, or Top Dead Center (TDC). In a four-stroke engine, TDC occurs twice per cycle. In different regions, the piston may be compressing or expanding the mixture, or, if a valve is open, moving the mixture into or out of the intake or exhaust manifolds.
Highlighted on the cylinder volume curve are two regions, one for when the exhaust valve is open and the other for when the intake valve is open. Note that the two valve events are separated by a number of crank angle degrees, termed Negative Valve Overlap or NVO. Unlike conventional engines, NVO prevents some of the hot exhaust gases from leaving the cylinder (typically 20-60%). This stores “residual gases” for the next cycle, offering a practical way to raise the mixture temperature to ensure HCCI autoignition. By changing the amount of NVO, one can affect the mixture temperature and dilution and ultimately control the chemical kinetics behind combustion timing. Temperature and dilution work in opposite directions, but typically temperature dominates. NVO is not instantly adjustable with common variable valve timing systems, and the reader is cautioned that many researchers publish results with fully variable (lift and timing) electric or hydraulic valve actuation systems that are expensive to implement in production engines.
The use of NVO residual gases introduces strong cycle-to-cycle coupling on top of the already non-linear chemistry and physics that occur throughout a complete engine cycle. Further compounding the issues with residual gases is that neither the airflow to the cylinder(s) nor the quantity of residual gases in the cylinder(s) can be accurately resolved before a burn happens on a cycle-to-cycle (not mean value) basis with commonly available sensors, especially during transients. Beyond residual gas influences, there are also complex secondary influences on combustion behavior such as turbulent mixing, manifold resonance effects, combustion deposits, different varieties of fuel and even ambient temperature variations.
While HCCI is already a significant challenge given the above complexity, the combustion mode also exhibits a period doubling bifurcation cascade to chaos, similar to what is seen in high residual spark ignition engines. When nearly chaotic, HCCI is still deterministic, but becomes oscillatory and very sensitive to parameter variations (e.g. residual gas fraction fluctuations). This oscillatory “stability limit” behavior is commonly referred to as high Cyclic Variability (CV) and it severely constrains the available load limits of HCCI.
This section provides background information related to the present disclosure which is not necessarily prior art.